1. Field of the Invention
The present invention relates to a method for zinc phosphating a molded metal article such as an automobile body, a household electrical appliance, steel furniture and the like.
2. Description of Related Art
In general, zinc phosphating is made to molded metal articles such as automobile bodies, household electric appliances, steel furniture and the like, prior to providing an overlay coating thereon. Spray and dipping methods are typically employed for such zinc phophating. The dipping method is preferably employed when a treated object has a baggy structure as an automobile body does and a higher corrosion resistance must be given to the treated object after the overlay coating is provided. The dipping method involves immersing the molded metal article into a treating bath of a treating solution in a treating tank.
The treating bath for zinc phosphating typically contains phosphate ions, zinc ions and other metal ions. Frequently, the treating solution further incorporates a chemical conversion accelerator for the purpose of accelatating formation of zinc phosphate coating. Examples of suitable chemical conversion accelerators include nitrite ion, hydrogen peroxide and nitrobenzensulfonate ion as has been conventionally used. Of the above, nitrite ion is widely used.
However, enviromental issues are accompanied by the use of nitrites such as sodium nitrite for the chemical conversion accelerator as nitrogen oxides evolve from the treating bath containing the same.
Japanese Patent Laying-open Nos. Sho 64-277 (1989) and Hei 5-195245 (1993) respectively disclose employing a treating bath containing hydroxylamine such as hydroxylamine sulfate for the chemical conversion accelerator. The use of hydroxylamine as the chemical conversion accelerator enables a chemical conversion treatment to be made without unfavorable evolution of nitrogen oxides. Similarly to conventional chemical conversion accelerators, it is also effective to inhibit formation of iron sludges in the treating bath, which advantageously permits a small-scale desludging process.
However, the use of hydroxylamine as the chemical conversion accelerator is not well-suited to treating metal articles having a baggy structure such as an automobile body and the like because it leads to an unsatisfactory formation of a zinc phosphate coating on the baggy structure and imparts poorer corrosion resistance characteristics thereto compared to the other types of chemical conversion accelerators.
Accordingly, it is an object of the present invention is to provide a method for zinc phosphating a molded metal article which is capable of inhibiting formation of nitrogen oxides and iron sludges and of enhancing corrosion resistance characteristics in the baggy structure of the article while solving the aforementioned problems as has been conventionally raised.